The present invention relates to lasers and, more particularly, to a system for determining the phase difference between mutually coherent light beams. Still further, the invention relates to synthetic aperture or phased array laser transmitters and an apparatus and method for determining whether adjacent transmitters are properly aimed and aligned.
In directing or receiving several coherent high energy laser light beams using synthetic aperture or phased array techniques, it is important to have all of the beams in phase. When using segmented mirrors, in far-term large aperture space sensors, for example, it is important that each segment or sub-assembly mirror be properly aligned to insure that the beams reflected from the mirror are in phase and directed to the proper focal point. Phased array laser transmitters have similar requirements. Proper alignment is accomplished by independent adjustment of each sub-assembly mirror and enables the segmented mirrors to perform as a single unit. Thus, proper alignment insures that each segmented mirror directs its beam in phase with the others for maximum intensity.
In such systems, a means is needed to determine whether or not the coherent beams are in phase and to adjust them if they are not. Optical path difference sensors can be used to determine the phase difference between coherent beams. Once the phase difference is determined between the beams, the beams can be adjusted so that they are in phase. Optical path difference sensors are known for sampling and determining the phase difference between coherent beams. Such sensors include image plane phase difference sensors and multiple order radial-grating shearing interferometers. In practice, these sensors have several disadvantages, some of which are as follows: When laser amplitude varies across the beam being sampled, as is often the case, the accuracy of present sensors suffers. These sensors require periodic calibration of the bias and the responsivity of the detectors to maintain accuracy. In addition, there is a need for sensor line of sight (LOS) stabilization to remain within the detector array field of view. Also, in the multiple order radial-grating shearing interferometer there is a need for a square wave reference signal, which is derived from a rotating square wave grating. Further, these sensors are highly complex and expensive.
Those skilled in the art may be aware of still other disadvantages in the use of the types of optical path difference sensors mentioned herein and common in the field.